The PR interval on a standard 12-lead ECG represents conduction through which cardiac structures in sequence?

Question

Accepted Answer

A. Atrial muscle → AV node → His bundle → Purkinje fibres. ## PR Interval Definition

The PR interval is measured from the **onset of the P wave to the onset of the QRS complex**.

### Anatomical Pathway

**Key Point:** The PR interval encompasses conduction through the following structures **in sequence**:
1. **Atrial muscle** — represented by the P wave; impulse spreads from the SA node across both atria
2. **AV node** — the major site of physiological conduction delay (~100 ms)
3. **His bundle** — rapid conduction through the common bundle
4. **Purkinje fibres** — rapid distribution to ventricular myocardium (the QRS begins as Purkinje fibres activate ventricular muscle)

> **Why not the SA node?** The SA node fires *before* the P wave begins; it is not included in the PR interval measurement, which starts at the **onset of the P wave** (i.e., onset of atrial depolarisation).

> **Why include Purkinje fibres?** The QRS complex begins only when ventricular *myocardial* cells start to depolarise. The His–Purkinje conduction itself occurs within the PR interval (isoelectric segment after the P wave), so Purkinje fibre conduction is correctly included in the PR interval.

### Normal Duration

**High-Yield:** Normal PR interval = **0.12–0.20 seconds (3–5 small squares)** on standard ECG paper (25 mm/s).

### Why Option B (SA node → atrial muscle → AV node → His bundle) is incorrect

The PR interval is measured from the **start of the P wave**, not from SA node firing. The SA node depolarisation is electrically silent on surface ECG and precedes the P wave; it is therefore **not part of the PR interval**. Additionally, option B omits the Purkinje fibres, which conduct the impulse to the ventricular myocardium within the PR interval.

### Clinical Significance

| Finding | Interpretation |
|---------|----------------|
| Prolonged PR (>0.20 s) | First-degree AV block |
| Short PR (<0.12 s) | Pre-excitation (e.g., WPW syndrome) |
| Progressive PR lengthening | Mobitz type I (Wenckebach) second-degree AV block |

**Clinical Pearl:** The AV node accounts for approximately 100 ms of the total PR duration and is the primary site assessed when the PR interval is prolonged.

[cite: Harrison 21e Ch 269; Guyton & Hall Medical Physiology 14e Ch 13]

![ECG — Waves and Intervals diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3488.webp)

Answer

B. SA node → atrial muscle → AV node → His bundle

Answer

C. AV node → His bundle → Purkinje fibres → ventricular muscle

Answer

D. Atrial muscle → AV node only

The PR interval on a standard 12-lead ECG represents conduction through which cardiac structures in sequence?

Explanation

PR Interval Definition

Anatomical Pathway

Normal Duration

Why Option B (SA node → atrial muscle → AV node → His bundle) is incorrect

Clinical Significance

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Finding	Interpretation
Prolonged PR (>0.20 s)	First-degree AV block
Short PR (<0.12 s)	Pre-excitation (e.g., WPW syndrome)
Progressive PR lengthening	Mobitz type I (Wenckebach) second-degree AV block